In conventional hydraulic or oleo-pneumatic shock absorbers, use is made of a system comprising a rod-and-piston and a return spring, which system is interposed between the structure that is to be protected (e.g. the bodywork of a motor vehicle) and a source of external disturbances (e.g. a wheel of the vehicle in direct contact with the ground). A cylinder and rod-and-piston unit is then provided that is surrounded by the return spring and that has the function of dissipating the energy of impacts by making use of the viscous flow of a hydraulic fluid. Energy is dissipated in conventional shock absorbers of that type by the solid-liquid system transforming the mechanical energy of friction into heat, which is given off to the outside.
Such conventional shock absorbers are very widespread, but they remain tied to a principle of energy dissipation that is obtained solely by throttling a viscous fluid, generally oil, which explains the poor dissipation power of such shock absorbers. There also exist practical drawbacks that are inherent to their structure, in particular the fact of always being under high pressure. This applies for Monroe shock absorbers (oleo-pneumatic shock absorbers from the inventor Bourcier de Carbon) that make use of a free floating piston between the gas and the oil. Even when the shock absorber is in the rest state, there exists a permanent pressure lying in the range 50 bar to 100 bar that is there to prevent the oil vaporizing when it passes through calibrated throttling orifices. The presence of such a high pressure can give rise to danger during assembly and disassembly, and also while the shock absorber is being handled. In order to avoid that danger, it is conventional practice to provide a safety band that goes in front of the free end of the rod-and-piston so as to avoid any sudden extension of the rod that could give rise to a severe accident.
Another drawback, which is likewise inherent to the fact of being permanently under high pressure, is that for storage or transport of the shock absorber, the rod-and-piston is in the extended position, such that the shock absorber occupies a long length.
About ten years ago, proposals were made to devise a new type of shock absorber capable of obtaining much greater energy absorption-dissipation power, while being structurally lighter and less bulky than conventional shock absorbers. In this context, reference may be made to document EP 1 250 539 B1, which has the same inventor as the present application.
That new type of shock absorber uses a concept of a heterogeneous energy absorption-dissipation structure using a porous capillary matrix and an associated liquid relative to which said matrix is lyophobic, as described in detail in document EP 0 791 139 B1, which has the same inventor and is ten years older still. In accordance with that very innovative type of heterogeneous structure, a porous capillary solid matrix is used having pores that are open and of controlled shape, together with a liquid surrounding the porous capillary matrix so as to define a large specific separation surface area between the solid and the liquid, with the matrix being lyophobic relative to the liquid. The separation surface area then varies isothermally and reversibly as a function of the external pressure to which the heterogeneous structure is subjected.
The isothermal “compression-expansion” cycle of the heterogeneous structure is characterized by a closed loop presenting a large amount of hysteresis in the PV diagram, where hysteresis H corresponds to the difference ΔP=Pint−Pexp, where Pint is the pressure for forced intrusion of the liquid into the pore space of the matrix, and Pexp is the pressure at which there is spontaneous expulsion of the liquid from said pore space, with the area defined by the closed loop characterizing the amount of energy that is dissipated. That fundamental principle is very innovative and is explained in detail in the publication that appears in the English journal J. Automobile Engineering, V. A. Eroshenko, 2007, Vol. 221, Part D, pp. 285-300 under the title “A new paradigm of mechanical energy dissipation—Part 1: theoretical aspects and practical solutions”, and pp. 301 to 312 under the title “A new paradigm of mechanical energy dissipation—Part 2: experimental investigation and effectiveness of a novel car damper”.
Document EP 1 250 539 B1 thus describes a shock absorber of the type comprising a rod-and-piston assembly slidable in a cylinder and defining on either side of the piston respective working chambers containing the hydraulic fluid, each working chamber communicating continuously with an associated chamber containing a heterogeneous energy absorption-dissipation structure, and also communicating with a common chamber via an associated system having a check valve and a constriction, the common chamber constituting a compensation chamber ensuring hydraulic fluid continuity during the movements of the rod-and-piston assembly in the cylinder. In this context, reference may be made to document EP 1 250 539 B1, which has the same inventor as the present application.
In that shock absorber, energy is dissipated without having recourse to the viscous fluid, e.g. oil, as soon as the travel speed of the piston exceeds a predetermined critical speed for switching from a conventional Newtonian regime to a surface-energy regime, making use of heterogeneous energy absorption-dissipation structures in which the “solid-liquid” interface acts as a working body.
Nevertheless, certain drawbacks have been found in the above-mentioned shock absorber structure.
Firstly, when the shock absorber uses a conventional two-chamber piston with two sealing systems, it is necessary to provide a body that extends towards the rear along a length that is sufficient to enable the rod-and-piston to penetrate completely, thereby giving rise to a shock absorber that is of considerable length, even when the rod-and-piston has penetrated completely.
Furthermore, the sole compensation chamber, which is arranged in the central portion of the shock absorber, is a chamber having a deformable wall defined by a flexible bag, and locating it in that position inevitably gives rise to a certain resistance to the transfer of heat between the working chambers and the outside.
Finally, the two flexible bags, each housing a respective heterogeneous energy absorption-dissipation structure constituted by at least one porous capillary matrix together with an associated liquid relative to which said matrix is lyophobic, are arranged in respective dedicated chambers of the two-chamber rod-and-piston. Consequently, those sealed bags are remote from the outside surface of the body of the shock absorber. Unfortunately, it is that surface which determines the effectiveness of heat exchange between the porous capillary matrices and the outside, and as a result a significant increase is observed in the temperature of said matrices in situations of severe operation and/or with high peak speeds of the rod-and-piston.
Document GB-A-1 188 453 discloses an oleo-pneumatic suspension having a tubular body defining a central chamber slidably receiving a piston and two annular chambers surrounding the central chamber. The central chamber is filled with oil and the piston is provided with channels allowing constrained passage of the oil from one side of the piston to the other. The annular chambers are separated by a deformable wall into two compartments, one containing oil and the other containing air. The oil-containing compartments are in communication with the central chamber via constrained passage channels, each on a respective side of the piston. It can be understood that the annular chambers provided with deformable walls separating oil and air perform a suspension function by compressing/relaxing the air in order to form a pneumatic spring.